The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In magnetic resonance imaging systems, gradient coil assemblies are commonly pulsed with electrical current pulses to produce magnetic gradients across a main magnetic field in the vicinity of an imaging region. The magnetic field gradients tend to interact with external metallic structures such as the magnet cold shields, the magnet dewar, and the like. This interaction generates eddy currents in the affected structures. These eddy currents, in turn, generate eddy magnetic fields which have a deleterious effect on the temporal and spatial quality of the magnetic field in the vicinity of the imaging region and, hence, in the resultant image quality.
To compensate for long term eddy currents (i.e., eddy currents with time constants that are not short compared to an echo planar imaging (EPI) readout time, for example, on the order of or longer than the EPI readout time), typically, pre-emphasis hardware shapes gradient pulses and/or applies pre-emphasis currents to the gradient coils to counter the eddy currents. To adequately compensate for the long term eddy currents, it is advantageous to have the pre-emphasis hardware correctly calibrated.
Previous methods for achieving the desired calibration relied on an eddy current generating pre-scan gradient pulse followed by free induction decay (FID) readouts. Information regarding the time constants of the induced eddy currents was then extracted from the FID signal. The time constants could then be used to measure and correct for long term eddy currents via calibration of the pre-emphasis filters and/or currents. While generally sufficient for their relative applications, the prior art techniques suffer from various drawbacks. A noteworthy drawback is that the results are not image based, and as such, they are difficult to interpret by persons without some advanced mathematical background. Moreover, they lack the ability to easily quantify the effect of the residual eddy currents on images.
One type of prior art technique employs a specialized probe or small phantom in its calibration method. See, for example, U.S. Pat. No. 4,698,591, incorporated herein by reference. The probe is initially offset from the isocenter of the imaging region to be positioned where the gradient to be compensated is non-zero. Drawbacks to this particular method are that the specialized probe or small phantom is not widely available and that it is manually repositioned in order to obtain localized spatial information. Other prior methods have addressed these issues by employing a larger phantom and exciting only selected cross-sections thereof via application of a slice select gradient. See, for example, U.S. Pat. No. 5,451,877, incorporated herein by reference. Such methods, however, suffer from their own drawbacks. In the case of a spherical phantom, the spurious gradients generated by the eddy currents are integrated over a whole disk and not merely the axis of interest. Additionally, for localized spatial information, multiple experiments on different selected slices are performed. Moreover, the employment of the slice selection gradient tends to complicate matters as it introduces eddy current effects of its own.
Other efforts to address eddy current problems include using an imaging technique know as diffusion-weighted echo planar imaging (DWI). Again, however, DWI has certain drawbacks associated therewith. Of note is the fact that the time constant range of eddy currents which may accurately be observed using DWI is limited by the duration of the diffusion-weighting lobes. Moreover, DWI images have signal-to-noise limitations induced by the diffusion weighting, and scans collected with varying B-values have different base signal-to-noise ratios which makes comparison of images difficult.
The present invention contemplates a new and improved technique for long term eddy current pre-emphasis calibration which overcomes the above-referenced problems and others.